Reactive developer for electrophotography

ABSTRACT

The invention is directed to a liquid developer composition for electrophotography in which the developer particles are reactively produced in situ in the carrier liquid by the reaction of an electrically insulative substance with an electrically conductive substance.

United States Patent 1 M etcalfe et al.

REACTIVE DEVELOPER FOR ELECTROPHOTOGRAPHY [75] Inventors: Kenneth A. Metcalle, Lockleys;

Alwin S. Clements, Largs Bay, both of Australia {73] Assignee: The Commonwealthof Australia,

Canberra, Australia [22] Filed: June 6, 1973 [21] Appl. No.: 367,402

[30] Foreign Application Priority lJata June 15, 1972 Australia 9341/72 52 us. Cl. 252/62.l [51] Int. Cl. I 603G 9/04 [58] Field of Search 252/621; 8/71 [56] References Cited UNITED STATES PATENTS 3,639,243 2/1972 Okuno et al. 252/621 Aug. 19, 1975 3,720,619 3/1973 lnoue et al 252/621 3,753,760 8/1973 Kosel 252/62.l OTHER PUBLICATIONS Giles et 211., Part l Journal of Chemical Society, 1952, pp. 3799-3805.

Arshid et al.,v Pait lI, Journal of Chemical Society, 1955, pp, 67-79.

Arshid et al., Part III, Journal of Chemical Society, pp. 72-75.

Primary Examir'zerNorman G. Torchin Assistant ExaminerJ. P. Brammer Attorney, Agent, or F irm-Waters, Schwartz & Nissen [5 7 ABSTRACT The invention is directed to a liquid developer composition for electrophotography in which the developer particles are reactively produced in situ in the carrier liquid by thereaction of an electrically insulative substance with an electrically conductive substance.

' 12 Claims, No Drawings REACTIVE DEVELOPER FOR ELECTROPHOTOGRAPHY STATE OF THE ART The earlier forms of development of electrographic images employed dry powderswhichwere applied to the surface of the sheet to be developed by a cascade method or as a powder cloud. A magnetic brush was later employed to carry the dry powder across the surface so that selective deposition could occur. Each of these methods involved the use of powders made by grinding or the like in air, and contamination of the surfaces of the powders during manufacture, storage and application could easily occur. A furthercharacteristic of the dry powder method was that the ir nage required fixing by heat fusion or the like after deposition and the powders thus required an admixture or attachment of a fusible medium such as resins.

where necessary, as well as controlled use of bi-polar particles and aggregates. Considerable work has been done'to achieve ready control of reversal of image deposition. i I

One of the best known methods was that called controlled development in which a substance was added tothe liquid developer which controlled, enhanced or retarded the migration of particles or aggregates to the image either to produce a negative, positive or neutral result.

In these liquid systems it is usual to disperse pigment particles, or dye,'or polymer particles or globulesin a liquid with relatively high electrical resistivity such as 10 ohm cm, and with a dielectric constant less than 3. The particles so dispersed will'deposit according to the inherent charges on the particles and also according to the dielectric constant of the particles because an induced charge may add to the total chargeor subtract from it. Many substances exist which will deposit under these conditions, but the introduction of a control sub? stance gives a common and a better deposition which can be regulated to give better results, and this'system is now in general use.

The mechanism for controlling the movement of particles in an electrically insulating liquid in the presence of a field is a complex one and many factors must be considered if the, best results are to be obtained. These factors include for example, the ionic double layer.

around the particles or aggregates, the extent of dielectrophoretic effects and the effect of. syneresis or the electrical interaction of particles on one'another. A further requirement is to consider'the conductivity of the toner particles. Another requirement is to consider particle size and concentration well as chemisorption effects. I:

,We have observed that one .of the most important factors in the production of developers for electrophotographic purposes is the nature of the pigment or toner particles and particularly whether their surfaces have been impaired, altered or contaminated prior to dispersion in the carrier liquid. Insofar as both electrophoresis and dielectrophoresis are concerned, a marked influence is exerted by the surface condition of the particles. In our controlled developer system we modified this situation by grinding the particlesto produce new fracturesurfaces within a primary dispersion medium such as a resin, oilor surfactant, to produce as many new surfaces away from the air or other oxidising environment.

SUMMARY OF INVENTION From the intensive research which we have carried outwe have shown that the use of pigments and the like made externally to thecarrier liquid itselfpresents many problems because of the difficulty of protecting the particles from unwanted contamination which of course may seriously impair particle migration and the end usepf the image deposit as for example by formation of electrical resists for overprinting or chemical resists for subsequent letterpress or lithographic-printing.

We found that these uncertainties could be eliminated by forming the particles within the liquid carrier and it is clear that any pigment, dye ortoner can be used if the components from which they are made can be introduced into the carrier liquid in solution or in a form precipitated within the carrier liquid.

Thus we propose the use of substances which can be introduced into a carrier liquid which interact, or combine with or result in decomposition products which have the desired properties of color, chemical response and the like in an electrographic context.

In an earlier filed application the substances which reacted were such that they themselves do not impair theresistivity of the carrier liquid. That is the ideal condition for such a developer, but we have now found that it is possible to combine a substance which itself is sufficiently insulating so as to not impair the resistivity of the liquid, with a substance which by itself would so lower the resistivity that development could no longer occur.

An object of the present invention is to make developer particles and aggregates within an insulating carrier liquid in order to exclude the possibility of oxidation or other unwanted contamination due to exposure to air or other contaminating gas, other objects being to make particles or aggregates within the carrier liquid so as to exclude the difficulties ordinarily encountered in wetting dry pigments and powders properly with resins, oils and the like so as to achieve control of their migration in the liquid when under the, influence of an electric field or when subjected to selective chemisorption by the surfaces on which they are placed, and also to associate the particles with adhesives or fixatives or chemically combining agents which enable the total-aggregates which are produced by the method to be fixed to the receiving surface.

Another object is to clean the carrier liquid prior to the developer formation.

DETAILS'OF THE INVENTION The objects are achieved by placing into a solvent, at least two substances one of which is a relative insulator and the other is a relative conductor, which sub inces are dissolved to form a fine developer complex by reaction. The solvent and complex so produced may be suspended in a carrier liquid which has the effect of forming crystals or aggregates to result in larger developer particles which will not contaminate the carrier liquid.

While the initial complex may be too fine to be suitable as a developer, compounded crystal or combined molecule which form are highly suitable as a developer.

The carrier liquid must of course have an electrical resistivity sufficiently high, such as ohm. centimetre, to insure that the latent electrostatic image, if such is being developed, is not destroyed duringdevelopment, and this is so with the basic solvent also, which of course can be the carrier liquid.

The invention thus comprises the formation of a particulate developer consisting of pigment material having a charge characteristic such that the particles formed by the combined dissolved substances form an effective developer, without the particles being themselves true insulators at the interface with the carrier liquid.

It is of interest to note at this stage that we have found certain advantages in forming developers which do not have a true insulating surface because a developer particle which is a true insulator, also has relatively low charge mobility and therefore such insulator particles will tend to retain a charge on their surface and when they are moved down to an area of opposite polarity by an induced field, they will tend to remain in position. This is because the charge exchange does not take place readily between the particles and the surface and charges thus can not be readily neutralized either in the particle or in the surface with the result that th particles tend to remain on the surface.

It has however been shown that there are some conditions where this low charge mobility is not necessarily an advantage and by means of the present invention it is-possible to build in a particular balance between insulation and conductivity of the particles so that the particles can be given a selective charge mobility such that they can be retained on a surface for a limited time.

Such developers can be very useful under conditions where for instance the developer is used with a bias field, and such a method of development has been previously described in an earlier application of ours in which however the field was reversed during development to attain an effect somewhat along the lines of the present invention, but of course by a different system of development, in that it was a field which was reversed during development whereas according to the present invention the developer is given sufficient conductivity that there can be a charge exchange during the time of development, and some particles can first be drawn down and can then be repelled when charge exchange takes place, but other particles can move down so that there is an agitation of the developer particles which during development tends to cause particles to go down and to then dislodge, but by maintaining a correct balance between conductivity and insulating value of the particles the rate of charge exchange can be so controlled that the most effective development according to the system in which the developer is used can be attained.

To give an idea of how the invention may be carried into effect a number of Examples will now be given which are however typical only of the developers which can be formed.

' EXAMPLE 1 A first substance is selected which does not impair the electrical resistivity of a selected insulating liquid when dissolved or contained therein, for example:- 1(a) QUlNALlZARlN (l,2,5,8- TETRA' HYDROX- YANTHRAQUINONE) l grrn.

This material is dissolved in Esso '100 Solvent in the proportions of l gram in l00 millilitres of Esso 100. A second substance is selected which does change the electrical resistivity of the insulating liquid when dissolved therein, for example:-

1(b) COBALT OCTOATE, 6% solution in mineral spirits,

l millilitre- The cobalt octoate is dissolved in the of Esso 100 together with the quinalizarin. A complex is formed by the molecular interaction of the cobalt oc-, toate with the quinalizarin. The resultant product is then dispersed in lsopar G, H or E to give a totalvolume of 500 millilitres. The suspension formed in this way can be used for the development of electrostatic.

images, or the solids may be concentrated without drying out for example by centrifuging or by electrostatic precipitation within the liquid. The concentrate may be re-dispersed when required in further lsopar G or other insulating liquid of similar solvent .power, The developer may be further improved by removing any excess cobalt octoate by washing the concentrate with lsopar G and filtering without permitting the filter cake to dry before re-dispersion in fresh lsopar G. The lsopar G should be purified by distillation before use. It will be appreciated that the quinalizarin can be tolerated in excess in the carrier liquid but the cobalt octoate when present in excess of the amount needed to form the complex has progressively greater effect on the charge dissipating properties of the carrier liquid. I

Esso 100" is a hydrocarbon solvent supplied by Esso Chemicals boiling range l56-l7lC aromatic content 98.9 flash point l00 F specific gravity 0.874.

lsopar G is a hydrocarbon solvent supplied by Esso Chemicals boiling range l58l77C aromatic con- V tent 0.20 flash point I03F specific gravity 0.750.

EXAMPLE 2 EXAMPLE 3 In Example 1, the quinalizarin is replaced by AN THRANILIC AClD (O-AMINOBENZOIC ACID) which is soluble in lsopar G carrier liquid, and a complex is formed by reacting the anthranilic acid with cobalt octoate or the like.

EXAMPLE 4 l, the quinalizarin is replaced by (1,4 BENZOQUINONE, TETRA- In Example CHLORANIL CH LORO- millilitres.

The following materials are used to make a developer:-

SYM-DIPHENYLCARBAZIDE (C H .NH.NH) .CO 1 gram This material is dissolved in Esso 100 (100 millilitres), and to this solution is added:-

5(b) COPPER NONYL PHTHALATE 0.5 gram A strongly coloured complex is produced which is then diluted to 500 millilitres with lsopar G or H.

lsopar H, is a hydrocarbon liquid with a boiling range l74l 9 1C), Aromatic content 0.20, and Flash point l20F.

EXAMPLE 6 In Example 5, ethylene glycol monobutyl ether is added to the Esso I millilitres of the ether to 100 ml Esso 100) to produce an intensified colour.

EXAMPLE 7 v In Example 5, the sym-diphenylcarbazide is replaced by diphenylthiocarbazone (dithizone).

EXAMPLE 8 In Example 1, the quinalizarin is replaced by hydroquinone dissolved in tetralin and a triple complex is produced by reaction with zinc, lead, manganese, or zirconium salts.

In Example 1, the cobalt octoate is replaced by ferric chloride dissolved in triethylphosphate.

EXAMPLE I l Tetrachlorophthalic anhydride is dissolved in Esso 100 in the porportions 1 part in 100 by weight and precipitated as a complex with metal soaps.

EXAMPLE 12 In Example 1 l, the Esso 100 is replaced by myristic acid, lauric acid or stearic acid asa solvent for the tetrachlorophthalic anhydride.

EXAMPLE 13 In Example I l, the anhydride is replaced by fumaric acid.

EXAMPLE 14 Naphthalene 2 sulphonic acid is dissolved in tetralin and reacted with metal soaps prior to dispersion in a suitable insulating carrier liquid such as lsopar G.

EXAMPLE 15 a benzoin oxime (cupron) is reacted with iron salts to form a complex.

EXAMPLE l6 CUPFERRON (Ammonium salt of N-nitroso phenyl hydroxylamine) is reacted in solution in benzophenone (phenyl ketone) with metal salts to form complexes.

EXAMPLE 17 Alizarin is reacted with metal salts to form a complex.

EXAMPLE l 8 The following dyes are reacted in solution for example ethylene glycol monobutyl ether with sumdiphenylcarbazide to produce complex colour changes and toner particles:-

Kiton Fast Red 4BLN (CIBA-GEIGY AUSTRALIA) (renamed Erio Red 4BL) is an insoluble acid azo dye, one of a general class of metal free azo pigments (i.e. pure azo organic compound without any metal or inorganic component).

Pigment No.: Acid Violet 41 Colour Index No.: 17080 Cresol Red:- is an ortho-cresolsulphonphthalein acidbase indicator dye B.D.I-I. (U.K.) D.H.A./ANAX AUST.

Disulphine Blue: is a triphenylmethane acid blue reversible oxidation reduction indicator dye. Colour Index No.: 42045 I.C.I.A.N.Z.

Brilliant Green 16546 is a triphenylmethane class dye. Colour Index No.: 42040, Basic Green 1 George T. Gurr, U.I(. London Erioglancine (Alphazurine) is a triphenylmethane acid blue reversible oxidation reduction indicator dye. C.I. No.: 42045 B.D.H. Laboratory Chemicals Division, U.K. (D.H.A./ANAX AUST.)

Z-(p-Dimethylaminostyryl l -ethylpyridinium Iodide one of a group of the Psendocyanine dyes. Eastman Organic Chemicals, Rochester, New York, U.S.A.

Benzopurpioriul (48), (Eclipse Red; fast scarlet; azamin 43; cotton red) is a red substantive indicator dye formed by combining naphthionic acid with diazo compound of ortho-toliudine sodium 0- tolidinediazo-bis (naphthylaminesulphonate) MERCK & CO., INC., RAHWAY, NJ. U.S.A.

Eosin (Y) is a tetrabromofluorescienadsorption indicator dye Colour Index No. 45380 Eastman Kodak, Rochester, New York, U.S.A.

Rhodamine (B) is a basic dye formed from the condensation of phthalic anhydride with m-aminophenol or its derivatives.

e.g. (C H i l. (C H 2 B.D.H. Laboratory Chemicals Division (U.K.) (D.H.A./ANAX AUSTRALIA) We claim:

1. A developer for electrophotography comprising an insulating carrier liquid having an electrical resistivity of at least ohm-centimeter and developer particles suspended in said carrier liquid and being electrically conductive so as to be movable in the carrier liquid under the influence of an electrical field, said developer particles being reactively produced in situ in said carrier liquid and being constituted by the reaction product of at least two substances both soluble in said carrier liquid, one being a substance which is electrically insulative relative to the carrier liquid and which does not reduce the electrical resistivity of the carrier liquid when dissolved therein, the other substance being a substance which is electrically conductive relative to the carrier liquid and reduces the electrical resistivity of the carrier liquid when dissolved therein, said substances forming, when in dissolved state in said carrier liquid said conductive developer particles, the relative quantities of said two substances being so selected that there is no surplus of the relatively conductive substance after reaction in the carrier liquid to insure that the resistivity of the carrier liquid is not lowered in its insulating value, said developer particles by virtue of being formed in said carrier liquid being shielded against oxidation and being held in said liquid until used as a developer, said carrier liquid being a hydrocarbon' liquid,-saidsubstance of greater insulating value being selected from the group consisting of: Quinalizarin, anthranilic acid, Alizarin, Hydroq uinone, Chloranil, syrn-diphenylcarbazide, diphenylthiocarbazide, Z-Furoic Acid, Tetrachlorophthalic anhydride, fumaric acid, Naphthalene -2-SulphonicAcid, Citral, Cinnomoldehyde and Diphenyliamine, said substance which is more conductive being selected from the group consisting of: cobalt octoate, the octoates and naphthenates of zinc, lead, manganese, copper, zirconium, iron and chromium, copper nonyl phthalate and ferric chloride.

2. A developer according to claim 1 further comprising a further solvent dissolved in said carrier liquid and also having an electrical resistivity in excess of 10 ohm centimetre but having a lowersolvent power than the carrier liquid whereby to increase the size of the developerparticles which are formed.

3. A developer according to claim 1 wherein at least one of said substances is in liquid form compatible with the said carrier liquid and reactive with the other substance when that substance is in a dissolved state.

4. A developer according to claim 1 wherein at least one of the said substances is dyed before dissolution in the carrier liquid to give a selected color.

5. A developer according to claim 1 wherein said carrier liquid is Esso and further comprising ethylene glycol monobutyl ether dissolved in the carrier liquid to produce an intensifiedcolor.

6. A developer according to claim 1 comprising a second solvent dissolved in said carrier liquid and also an electrical insulator but of lower solvent power than said carrier liquid whereby to increase the size of the developer particles which are formed.

7- A method for making a developer for electrophotography of the type comprising an insulating carrier liquid having conductive developer particles suspended therein capable of being moved in an electrical field, said method comprising reactively producing the developer particles by placing into a carrier liquid having an electrical resistivity in excess of 10 ohm. centimeter, at least two substances both soluble in the said carrier liquid, one being a substance which is an insulator relative to'the carrier liquid and which does not reduce the electrical resistivity of the carrier liquid when dissolved therein, the other being a substance which is a conductor in relation to the carrier liquid and which reduces the electrical resistivity of the carrier liquid when dissolved therein, said substances being so selected to form, when in a dissolved state in the carrier liquid, another solution which precipitates to form said developer particles, said substances being selected in quantity such that by reaction therebetween there is no surplus of the conductive substance after reaction in the carrier liquid whereby the resistivity of the carrier liquid in its final state is not lowered in insulating value, and whereby the developer particles are themselves formed in the liquid to be shielded against oxidation andare held in such liquid until used as a developer, said carrier liquid being a hydrocarbon liquid, and said insulating substances being selected from the group consisting of: Quinalizarin, anthranilic acid, Alizarin, Hydroquinone, Chloranil, sym-diphenylcarbazide, diphenylthiocarbazide, 2-Furoic Acid, Tetrachlorophthalic anhydride, fumaric acid, Naphthalene2- Sulphonic Acid, Citral, Cinnomoldehyde and Diphenyliamine, said conductive substance being selected from the group consisting of cobalt octoate, the octoates and naphthenates of zinc, lead, manganese, copper, zirconium, iron and chromium, copper nonyl phthalate and ferric chloride.

8. A method according to claim 7 comprising diluting the conductive developer so formed with a further solvent also having an electrical resistivity in excess of l() ohm. centimeter but having a lower solvent power than the carrier liquid in order to increase the size of the developer particles which are formed.

9. A method according to claim 8 wherein at least one ofsaid substances is in liquid form compatible with said carrier liquid and reactive with the other substance when that substance is in a dissolved state.

10. A developer according to claim 8 comprising dyeing at least one of the said substances before dissolution in the carrier liquid to give a selected color. 

1. A DEVELOPER FOR ELECTROPHOTOGRAPHY COMPRISING AN INSULATING CARRIER LIQUID HAVING AN ELECTRICAL RESISTIVITY OF AT LEASST 10**10 OHM-CENTIMETER AND DEVELOPER PARTICLES SUSPENDED IIN SAID CARRIER LIQUID AND BEING ELECTRICALLY CONDUCTIVE SO AS TO BE MOVABLE IN THE CARRIER LIQUID UNDER THE INFLUENCE OF AN ELECTRICAL FIELD, SAID DEVELOPER PARTICLES BEING REACTIVELY PRODUCED IN SITU IN SAID CARRIER LIQUID AND BEING CONSTITUTED BY THE REACTION PRODUCT OF AT LEAST TWO SUBSTANCES BOTH SOLUBLE IN SAID CARRIER LIQUID, ONE BEING A SUBSTANCE WHICH IS ELECTRICALLY INSULATIVE RELATIVE TO THE CARRIER LIQUID AND WHICH DOES NOT REDUCE THE ELECTRICAL RESISTIVITY OF THE CARRIER LIQUID WHEN DISSOLVED THEREIN, THE OTHER SUBSTANCE BEING A SUBSTANCE WHICH IS ELECTRICALLY CONDUCTIVE RELATIVE TO THE CARRIER LIQUID AND REDUCES THE ELECTRICAL RESISTIVITY OF THE CARRIER WHEN DISSOLVED THEREIN, SAID SUBSTANCES FORMING, WHEN IN DISSOLVED STATE IN SAID CARRIER LIQUID SAID CONDUCTIVE DEVELOPER PARTICLES, THE RELATIVE QUANTITIES OF SAID TWO SUBSTANCES BEING SO SELECTED THAT THERE IS NO SURPLUS OF THE RELATIVELY CONDUCTIVE SUBSTANCE AFTER REACTION IN THE CARRIER LIQUID TO INSURE THAT THE RESISTIVETY OF THE CARRIER LIQUID IS NOT LOWERED IN ITS INSULATING VALUE, SAID DEVELOPER PARTICLES BY VIRTURE OF BEING FORMED IN SAID CARRIER LIQUID BEING SHIELDED AGAINST OXIDATION AND BEING HELD IN SAID LIQUID UNTIL USED AS A DEVELOPER, SAID CARRIER LIQUID BEING A HYDROCARBON LIQUID, SAID SUBSTANCE OF GREATER INSULATING VALUE BEING SELECTED FROM THE GROUP CONSISTING OF: QUINALIZARIN ANTHRANILIC ACID, ALIZARIN, HYDROQUINONE, CHLORANIL, SYMDIPHENYLCARBAZIDE, DIPHENYLTHIOCARBAZIDE, 2-FUROIC ACID, TETRACHLOROPHTHALIC ANHYDRIDE, FUMARIC, AND NAPHTHALENE -2-SULPHONIC ACCID, CITRAL, CINNOMOLDEHYDE AND DIPHENYLIAMINE, SAID SUBSTANCE WHICH IS MORE CONDUCTIVE BEING SELECTED FROM THE GROUP CONSISTING OF: COBALT OCTOATE, THE OCOATES AND NAPHTHENATES OF ZINC, LEAD, MANGANESE, COPPER, ZIRCONIUM, IRON AND CHROMIUM, COPPER NONYL PHTHALATE AND FERRIC CHLORIDE.
 2. A developer according to claim 1 further comprising a further solvent dissolved in said carrier liquid and also having an electrical resistivity in excess of 1010 ohm centimetre but having a lower solvent power than the carrier liquid whereby to increase the size of the developer particles which are formed.
 3. A developer according to claim 1 wherein at least one of said substances is in liquid form compatible with the said carrier liquid and reactive with the other substance when that substance is in a dissolved state.
 4. A developer according to claim 1 wherein at least one of the said substances is dyed before dissolution in the carrier liquid to give a selected color.
 5. A developer according to claim 1 wherein said carrier liquid is Esso 100 and further comprising ethylene glycol monobutyl ether dissolved in the carrier liquid to produce an intensified color.
 6. A developer according to claim 1 comprising a second solvent dissolved in said carrier liquid and also an electrical insulator but of lower solvent power than said carrier liquid whereby to increase the size of the developer particles which are formed.
 7. A method for making a developer for electrophotography of the type comprising an insulating carrier liquid having conductive developer particles suspended therein capable of being moved in an electrical field, said method comprising reactively producing the developer particles by placing into a carrier liquid having an electrical resistivity in excess of 1010 ohm. centimeter, at least two substances both soluble in the said carrier liquid, one being a substance which is an insulator relative to the carrier liquid and which does not reduce the electrical resistivity of the carrier liquid when dissolved therein, the other being a substance which is a conductor in relation to the carrier liquid and which reduces the electrical resistivity of the carrier liquid when dissolved therein, said substances being so selected to form, when in a dissolved state in the carrier liquid, another solution which precipitates to form said developer particles, said substances being selected in quantity such that by reaction therebetween there is no surplus of the conductive substance after reaction in the carrier liquid whereby the resistivity of the carrier liquid in its final state is not lowered in insulating value, and whereby the developer particles are themselves formed in the liquid to be shielded against oxidation and are held in such liquid until used as a developer, said carrier liquid being a hydrocarbon liquid, and said insulating substances being selected from the group consisting of: Quinalizarin, anthranilic acid, Alizarin, Hydroquinone, Chloranil, sym-diphenylcarbazide, diphenylthiocarbazide, 2-Furoic Acid, Tetrachlorophthalic anhydride, fumaric acid, Naphthalene-2-Sulphonic Acid, Citral, Cinnomoldehyde and Diphenyliamine, said conductive substance being selected from the group consisting of: cobalt octoate, the octoates and naphthenates of zinc, lead, manganese, copper, zirconium, iron and chromium, copper nonyl phthalate and ferric chloride.
 8. A method according to claim 7 comprising diluting the conductive developer so formed with a further solvent also having an electrical resistivity in excess of 1010 ohm. centimeter but having a lower solvent power than the carrier liquid in order to increase the size of the developer particles which are formed.
 9. A method according to claim 8 wherein at least one of said substances in in liquid form compatible with said carrier liquid and reactive with the other substance when that substance is in a dissolved state.
 10. A developer according to claim 8 comprising dyeing at least one of the said substances before dissolution in the carrier liquid to give a selected color.
 11. A method according to claim 7 wherein the carrier liquid is Esso 100 further containing ethylene glycol monobutyl ether to produce an intensified color.
 12. A method according to claim 7 comprising adding the developer so formed to a second solvent which is also an insulator but of lower solvent power than the carrier liquid whereby to increase the size of the developer particles. 